1. Field of the Invention
The present invention relates generally to air supply control systems for internal combustion engines, and more particularly to a system for controlling air supply effected to an internal combustion engine by a turbosupercharging device having variable supercharging capacity and accompanying with the internal combustion engine.
2. Description of the Prior Art
In an internal combustion engine equipped with a turbosupercharger having a turbine driven to rotate by exhaust gas flow from the engine, and a blower coupled with the turbine to rotate together with the same, a superior response in supercharging would not be expected in the case where the turbosupercharger is arranged to be relatively large in supercharging capacity and further it would not be expected that each of combustion chambers in the engine is sufficiently supercharged as occasion demands in the case where the turbosupercharger is arranged to be relatively small in supercharging capacity so as to have an improved response in supercharging.
In view of the above, there has been proposed a so-called sequentially controlled supercharging system in which a couple of turbosuperchargers of primary and secondary are provided to an internal combustion engine and so controlled that only the primary turbosupercharger works for supercharging the engine when intake air mass flow in an intake passage of the engine is relatively small and both the primary and the secondary turbosuperchargers work simultaneously for supercharging the engine when the intake air mass flow is relatively large, as disclosed in, for example, the Japanese patent applications published before examination under publication numbers 56-41417 and 59-160022. In such a system, the primary and secondary turbosuperchargers are caused to work selectively for varying their supercharging capacity to be reduced when the engine operates at a relatively low speed and to be increased when the engine operates at a relatively high speed, so that a superior response in supercharging can be obtained even on the occasion of engine operation at the relatively low speed and each of combustion chambers in the engine can be sufficiently supercharged on the occasion of engine operation at the relatively high speed.
In case of an internal combustion engine equipped with a single turbosupercharger with constant supercharging capacity, the turbosupercharger is operative to keep its rotation without reducing immediately due to inertia acting thereon when a shifting-up operation is performed in a transmission which is coupled with the engine and thereby the engine speed is reduced. Therefore, reduction in torque produced by the engine is suppressed on the occasion of the shifting-up operation.
However, in the engine which is provided with the primary and secondary turbosuperchargers controlled sequentially as described above, when the shifting-up operation is performed in the transmission so as to reduced the engine speed under a condition in which primary and secondary turbosuperchargers operate simultaneously with increased supercharging capacity, the primary and secondary turbosuperchargers are caused to reduce the supercharging capacity in response to the reduction of the engine speed and therefore torque produced by the engine is steeply reduced undesirably to cause torque shock in the engine and transmission. Especially, in the case of an engine coupled with an automatic transmission system, such torque shock as mentioned above is apt to be conspicuous.
This problem is generally encountered with an internal combustion engine provided with a previously proposed air supply control system including a supercharging device which is operative to work with supercharging capacity varying to be reduced when the engine operates at a relatively low speed and to be increased when the engine operates at a relatively high speed.